The Source engine is a 3D game engine developed by Valve Corporation. Its unique features include a large degree of modularity and flexibility, an artist-driven, shader-based renderer, accurate lip sync and facial expression technology, and a powerful, efficient and completely network-enabled physics system.
one of the best engines that i have EVER seen... It seens that this is recent but actually its from 2002-2004... It has been developed along with half-life 2 (interesting fact is that hl2 started its development back in 1999, but by that time valve was still using the goldsrc engine)
Despite it's age, the source Engine is an ever-expanding, fully functional game engine with many perks and capabilities that I feel make it an excellent choice for Game Developers and Modders. While it lacks some of the qualities that other Engines posses (such as DX10+ Support and Large-Scale worlds), Valve's continued efforts to expand upon the Engine to insure that it remains compatible and glitch-free are certainly one of the many reasons why for 6 years and counting, people from all across the globe are playing and modding the games blessed with the power of Source.
I hope Valve never lets go of this one. Great graphics, sound, effects, and online support.